A common technique for extracting metal from ores and other mineral material is to heap leach the material. An engineered pile of fragmented, un-consolidated rock, or heap, of particulate material is constructed, typically over an engineered liner and liquid collection system. A leach solution is applied to and percolated through the heap to contact the material and dissolve one or more metal of interest into the leach solution. Sprinklers are occasionally used for irrigation of the heap, but drip irrigation is more commonly used to minimize evaporation and more uniformly distribute the leaching solution. The solution, called a “barren solution”, containing metal dissolving lixiviants, percolates through the heap and leaches the target mineral or substance as well as other materials. This process, called the “leach cycle” can take between a couple of days to months or years depending on the material being leached.
Theoretically, the barren solution is supposed to travel vertically through the heap in a fairly uniform manner from the starting point of each drip point. In reality, within a relatively short period of time, a path of least resistance will be formed in the heap starting at each drip point that is based on the formation or placement of the material underneath the drip point. Each path of least resistance is unlikely to be vertical and as a result, large sections of the heap may receive no barren solution after a period of time, resulting in relatively little or no leaching of the target material. In addition, the leach solution may not uniformly contact all portions of the heap because of permeability variations that exist within the heap. Such permeability variations may result in preferential flow of leach solution through higher permeability portions of the heap, leaving lower permeability portions under-leached or un-leached. Also, the chemical properties in some portions of the heap may be less conducive to dissolution of the metal into the leach solution. For example, when heap leaching gold with a cyanide leach solution under alkaline conditions, low pH spots within the heap may not respond well to the alkaline leach solution, leaving those portions under-leached or un-leached. Metals remaining in under-leached and un-leached portions of a heap following heap leach operations often represent a significant loss to a mining operation.
A collection system collects the resulting pregnant leach solution drained from the liner, which is then processed to recover the dissolved metal. Once the target material has been removed from the pregnant solution, the once again barren solution, with additional lixiviants added, can then be reused in the heap leach process or treated further to remove certain toxic chemicals.
As noted, a common problem with heap leaching is the non-uniform leaching of metals from the heap. Even after extensive leaching over time, some portions of the heap may remain under-leached or even substantially un-leached. U.S. Pat. No. 8,021,461 describes a method for addressing non-uniform leaching in a heap that involves geophysically surveying the heap to identify portions of the heap that require further extraction, hydraulically fracturing the identified portions, and then treating the identified portions.